Revert of Remove media::VideoRenderer::SetPlaybackRate().

Revert of Remove media::VideoRenderer::SetPlaybackRate(). (https://codereview.chromium.org/384943002/)

Reason for revert:
Sorry to revert this change. It made the following layout tests flakily fail:
media/track/track-cue-rendering-horizontal.html
media/track/track-cue-rendering-vertical.html

Reverting locally healed the tests.

The image diffs show what appears to be a timestamp change (something like 0:18 to 0:19):
https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Linux__dbg_/20779/layout-test-results/results.html

Original issue's description:
> Remove media::VideoRenderer::SetPlaybackRate().
> 
> Video renderers don't need the information as they are already making
> audio/video synchronization decisions based on a media timeline that
> already incorporates the current playback rate via the time callback.
> 
> In particular, VideoRendererImpl only used playback rate to estimate
> a better duration to sleep until the current frame was ready ...
> except that in most cases we'd sleep for kIdleTimeDelta and wait
> until the media timeline had progressed past the current frame's
> timestamp. In other words, there's absolutely no reason to even try
> to estimate the sleep duration based on the playback rate.
> 
> BUG=370634
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282797

TBR=xhwang@chromium.org,scherkus@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=370634
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282891

[falken, 2014-07-14 02:46:44]

Created Revert of Remove media::VideoRenderer::SetPlaybackRate().

[falken, 2014-07-14 02:47:38]

The CQ bit was checked by falken@chromium.org

[commit-bot: I haz the power, 2014-07-14 02:48:36]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/falken@chromium.org/390733002/1

[commit-bot: I haz the power, 2014-07-14 02:49:00]

Change committed as 282891